Strike the Blood: Valkyria no Oukoku-hen

Kojou and Yukina are on their way to school as usual when they find La Folia being chased by someone. Yukina crosses blades with the enormous man who's pursuing her, but before the fight can escalate further, La Folia makes a shocking comment: "Father, this is the Fourth Progenitor, Akatsuki Kojou... my first!" La Folia's father, Lucas, had told her to decide who she wished to marry, so she declares that Kojou is her choice. Lucas refuses to accept this, but then La Folia's mother, Polifonia, invites Kojou and Yukina to a party so they can all talk in more detail. When they arrive at the party, they discover that Asagi, Yaze, Nagisa, and Sayaka have been invited as well, due to their connections with Kojou. What sort of ...